The invention relates to the modification of polymers by grafting hindered C-nitro compounds onto the polymeric substrate.
There is increasing concern about the migration and loss of additives from polymers. The additives, such as antioxidants, stabilizers, plasticizers, antistatic agents, photosensitizers and the like, may be lost from the polymer by volatilization or leaching. Once these additives are lost, they no longer can accomplish their intended purpose. Secondly, when such additives are leached into the surrounding media with which they are in contact, such as foodstuffs or other materials which may be ingested by humans or animals, they may cause danger to the ingestor since some of the materials are toxic in nature. Thirdly, when polymers are employed in medical applications, for example, in surgical goods, prostheses or body implants, it is essential that no migration of additives occurs since the additives are generally not biologically inert and thus they may cause toxicity problems in this environment as well.
Traditionally, additives, and particularly polymer stabilizers, have been relatively low molecular weight materials of high volatility and marginal compatibility with the base polymer. Recently, it has been proposed to overcome the problem of additives migrating out of polymeric materials by copolymerizing antioxidants containing vinyl groups with a major proportion of conventional monomers to give polymers containing the appropriate antioxidant in polymerized form at the required concentration. However, this is an expensive procedure.
It has also been proposed in U.S. Pat. No. 4,354,007 to Scott (hereinafter the '007 patent), that a wide variety of antioxidants and stabilizers may be reacted with preformed polymers in the presence of free radicals to provide a stabilized polymer directly or to make an intermediate, highly concentrated polymer-bound adduct which may be blended with further amounts of a suitable compatible base polymer to give an antioxidant-modified polymer suitable for a wide variety of applications.
The '007 patent describes a wide variety of antioxidant and stabilizer molecules including compounds selected from categories which include chain-breaking and/or peroxide-decomposing antioxidants, ultraviolet screening agents, triplet quenchers and metal deactivators. In terms of a limitation on the scope of the stabilizer compounds that may be used, perhaps the most significant structural limitation is that it should be capable of being activated by a free radical in the polymer (col. 1, lines 40-50). In this regard, however, it has been found with respect to the unsaturated vinyl group-containing stabilizer compounds described in the '007 patent and also with respect to similar stabilizer compounds which have been suggested by others for grafting to pre-formed polymers in the presence of free radicals, that frequently the reactivity of the stabilizer molecules containing polymerizable groups is such that the stabilizer tends to homopolymerize with itself to form a polymer or oligomer. Such a polymer or oligomer may, of course, have increased molecular weight as compared with the individual stabilizer molecule and this increased weight may inhibit undesired volatilization, and/or migration of the stabilizer in the polymer composition.
On the other hand, homopolymerization or oligomerization of the additives generally results in decreased activity of the additive in the polymer system. Thus, antioxidants and stabilizers bound to the polymer in this manner generally show overall lower stabilizing activity when compared with polymer systems containing a comparable amount of grafted antioxidant or stabilizer molecules. Further, the lower molecular weight polymers formed by homopolymerization of the additive are readily extracted from the polymer by extracting media.
Finally, U.S. Pat. No. 4,743,657 discloses a method for preparing polymer bound stabilizer which comprises reacting a stabilizer molecule containing a reactive double bond with a preformed polymer in the presence of a free radical. However, this patent does not disclose C-nitro compounds or that C-nitro compounds can be bound to polymer substrates by the method of the present invention.
Although many attempts have been made to produce nonextractable antioxidants and stabilizers by polymerizing, copolymerizing or grafting of antioxidants containing polymerizable vinyl groups, few commercial products are available in spite of the substantial activity documented in the patent literature. The reasons for lack of commercial success with the prior art methods are essentially:
(i) Homopolymerized antioxidants are incompatible with other polymers and, consequently, have low antioxidant activity. PA1 (ii) Copolymers of vinyl antioxidants and normal monomers, although oxidatively stable, are much more expensive to manufacture than conventional large tonnage plastics such as polyethylene, polypropylene, polyvinylchloride and polystyrene since the scale of manufacture is much reduced. No new oxidatively stable plastics based on these monomers are believed to be in commercial production. PA1 (iii) Grafting of vinyl antioxidants and stabilizers onto preformed polymers has been widely reported, but no commercial products have been produced since the efficiency of the binding process is generally low and the products so produced are not sufficiently effective to justify the cost of the modification procedure.
The cost of modifying all the polymer substrate can, in principle, be avoided by carrying out the modification process in such a way as to produce a concentrated masterbatch of bound antioxidant which can subsequently be used as a normal additive for polymers during the usual polymer processing procedure. This method has been used previously for thiol adducts to the double bond in rubbers, but it much more difficult to employ with saturated polymers due to the inefficiency of the grafting process referred to above. An object of the present invention is to obviate or mitigate the aforesaid disadvantages for C-nitro compounds.
Some of the hindered C-nitro compounds used in the process of the present invention are known, but their use in the method of the present invention has not been suggested before.
The Journal of Applied Polymer Science Vol. 12, pp. 1683-1693 (1968) discloses certain nitro-alkyl acrylates and methacrylates such as 2-methyl-2-nitro-2,3-propylene diacrylate. In this publication homopolymers and copolymers with styrene were tested as fuel binders of solid rocket propellants. No mention is made of the process of the present invention for use with these compounds.
U.S. Pat. No. 4,007,159 discloses hindered phenolic antioxidants such as 2,6-di-t-butyl-4-(2-methyl-2-nitro-propyl) phenol. No mention is made of the use of such compounds in a process like that of the present invention.
U.S. Pat. No. 3,650,984 discloses the reaction products of nitroalkanes with organo tin compounds. According to the disclosure, however, the complexes obtained are used in polyvinylhalide resin compositions for stabilization against degradation by heat and the compounds are not employed in a grafting process like that of the present invention.
DD-124 054 discloses the use as light stabilizers, vinylic nitro compounds having the general formula: ##STR1## wherein R.sub.1, R.sub.2 and R.sub.3 are chosen from the group of hydrogen or (un)substituted alkyl-, cycloalkyl-, aralkyl-, aryl-, amino-, mercapto-, carbonalkyl-, carboxy-, or heterocyclic groups. However, this patent does not disclose the grafting process of the present invention.